Epidemiological studies have indicated that some human skin tumors may be a consequence of exposure to the ultraviolet wavelengths of sunlight. Likewise, chronic ultraviolet irradiation induces highly antigenic spindle cell skin tumors on the back of inbred mice. This tumor system offers a unique model to study the lymphocyte mediated suppressive effects of ultraviolet light (UVL) as well as anti-tumor immunity in these hosts. The majority of tumors induced by UVL are immunogenic and, thus, rapidly rejected when transplanted into syngeneic hosts; however, UVL-induced tumors grow progressively in autochthonous and syngeneic UVL-irradiated hosts. Suppressor T cell activity and not panimmunosuppression has been established as the mechanism for this tumor progression. Through the use of established in vivo and in vitro methodology studies will be undertaken to determine the basic immune response to UV light-induced murine skin tumors. The primary objective is to gain a better understanding of how tumors regress or progress by studying the tumors in situ. The infiltrating host cells will be analyzed as to type by identification of cell surface markers and as to function by measuring cytotoxicity to tumor targets. A comparison will be made in regressor and progressor mice of both normal and UVL-irradiated origin. The interactions of infiltrating host cells with each other and within the local tumor environment will be analyzed. The relationship between peripheral anti-tumor reactivity and the immunologic response in situ will be determined.